Photocurrent multiplication has been observed in a hydrogenated amorphous silicon-based p-i/a- SiN/i-n multi ayered heterojunction under a reverse biased condition. A systematic investigation on the photocurrent characteristics in this junction system has been carried out, including photocurrent-voltage characteristics, light intensity and operation temperature dependences, spectral dependence and transient response characteristics. It has been found from the analysis of the results that multiplication arises from the interband tunneling injection of valence band "electron" through the a-SiN barrier layer. The photocurrent multiplication process is modeled to be comprised of three key elemental processes occurring sequentially in time: (a) accumulation of holes at the a-SiN/a-Si heterojunction interface, (b) field redistribution over the heterojunction, and (c) interband tunneling of carriers via the localized states in the a-SiN layer's energy gap. The device modeling on the basis of the experimental data permits us to design the device structure for achieving better device performances. By an optimization of device structure, an external quantum efficiency exceeding 70 with a response time as fast as 500 mus has been obtained under the operation voltage of 30V in the heterojunction photodiode provided with a-SiN (thickness of 40nm with optical energy gap 2.1 eV) at the p a-SiC/i a-Si interface. The proposed highly sensitive photomultiplier device would have a wide variety of application fields such as a solid-state imager for high-definitive television, and so on.